1. Field of the Invention
The present invention relates to a transmitter having particular, but not exclusive, application in cellular telephones operating in accordance with or based on standards such as GSM (Global System for Mobile Communications). For convenience of description the present invention will be described with reference to the GSM 900 Standard.
The transmitter may be the transmitting part of a transceiver and in the present specification and claims xe2x80x9ctransmitterxe2x80x9d is intended to cover a stand alone transmitter and a transmitting part of a transceiver.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings respectively show the transmitter noise and modulation specification for GSM 900 and a spectral template for modulation. FIG. 1 indicates the permitted levels of unwanted noise in a 1 Hz bandwidth referenced to a carrier power level of +33 dBm. In the region of the GSM receive band, Rx Band, between 935 MHz and 960 MHz, the noise must be held below xe2x88x92162 dBc and between 925 MHz and 935 MHz it must be below xe2x88x92150 dBc. Such low noise levels are not easy to achieve, especially with a fully integrated transmitter.
FIG. 2 illustrates the spectral template showing the modulation spectrum and limits on phase noise close to the carrier indicated by the zero frequency. Between 400 kHz and 1800 kHz, and between xe2x88x92400 kHz and xe2x88x921800 kHz, it desirable to have a margin of say 10 dB between the template and the floor of the phase noise. In order to be able to meet this requirement, the error in the phase trajectory for the modulation at an output of the transmitter VCO (voltage controlled oscillator) must be substantially less than 5xc2x0 rms and 20xc2x0 peak for 148 bits of random data.
An object of the present invention is to provide a transmitter architecture which is able to achieve predetermined transmitter noise and modulations specifications and to be capable of being integrated.
According to the present invention there is provided a transmitter comprising a phase modulator and a phase locked loop having a relatively high powered voltage controlled- oscillator (VCO), wherein the phase modulator comprises a reference frequency source and means for deriving phase modulation signals by random interpolation between at least 2 phase components derived from the reference frequency source.
The present invention provides a transmitter comprising a phase modulator and a phase locked loop having a relatively highpowered voltage controlled oscillator (VCO) and a phase sensitive detector having an input coupled to an output of the phase modulator, the phase modulator comprising a reference frequency source,.means for deriving at least 2 phase components of the reference frequency produced by said source and phase selection means controlled by a modulator for deriving a phase modulating signal by random interpolation between the at least 2 phase components.
An embodiment of the present invention uses a fixed frequency stable reference oscillator whose output is split into four quadrature phase components of equal amplitude. A single pole, 4-way tap selector selects any one of the four quadrature phase components for delivery to the phase locked loop (PLL). To obtain precisely the required instantaneous phase, the tap selector is made to hop randomly and quickly between the four quadrature phase components in what is essentially a process of interpolation. By hopping quickly, any noise is shifted outside the band of wanted frequencies and is removed by loop filtering. The switching is conducted under the control of a complex, 2 bit, bitxe2x80x94stream modulator such that the average position of the tap selector corresponds to the value of the phase required and the energy in the quantisation noise is translated to a high frequency, outside the bandwidth of the PLL.
This architecture produces very low levels of noise close to the carrier and is tolerant to integrated circuit process variations. It is simple, flexible and easily realisable method of applying modulation to an IF carrier.